The present invention relates to measuring the absolute static pressure of fluid flowing in a microfluidic device.
Conducting of fluid in microfluidic devices is frequently done in inkjet printers. It is also done in other types of microfluidic devices in fields not related to printing, for example, drug delivery and microscale biological and chemical analysis. In the inkjet printing field, ink is transported from a reservoir to a printhead, where it is sprayed onto print media. If a nozzle becomes clogged, the absolute static pressure within the relevant ejection chamber may exceed normal values. Thus, absolute static pressure detection can be used as a nozzle-by-nozzle diagnostic tool in the case of an inkjet printer. Similarly, absolute static pressure sensing in drug delivery systems is critical due to the potential hazards of inaccurate dosing. In microscale biological and chemical analysis systems, where precision is critical, an integrated absolute static pressure sensor reduces the error associated with sensing the absolute static pressure across an intervening volume between the microfluidic device and a non-integrated sensor; in addition, an integrated pressure sensor reduces the volume of fluid that must be sacrificed to measure the pressure. This increases cost and waste, and in the case of biological analysis systems, for example, it increases the required sample size, making sample collection and preparation more difficult and costly.
While prior art exists for pressure drop measurement across microfluidic and macrofluidic systems, even microscale measurement systems are integrated on the macroscale, allowing measurement of absolute static pressure as the working fluid enters or exits a microfluidic device, but not while the working fluid is within the interior of the microfluidic device. For example, U.S. Pat. No. 4,426,768 teaches the use of the measurement of a strain-dependent property to deduce the local absolute pressure of a non-conductive fluid. Similarly, U.S. Pat. No. 4,463,336 teaches the measurement of absolute gas pressure using the varying resistance of a narrow piezoresistive bridge. Electrically conductive liquids cannot be measured with this sensor. U.S. Pat. No. 4,682,503 teaches the use of measuring the varying thermal resistance of a silicon nitride bridge to the same end. This sensor does not work with an electrically conducting fluid, and struggles to provide accurate data with a fluid of high heat capacity. A number of sensors measuring pressure via the capacitance between a deforming membrane and a fixed electrode have been previously disclosed. U.S. Pat. Nos. 5,305,643; 5,316,619; 5,369,544 and 6,109,113 teach the measurement of absolute pressure against a standard (normally vacuum or the ambient atmosphere) by capacitance, while U.S. Pat. Nos. 5,332,469; 5,679,902 and 6,012,335 teach the use of capacitance to measure the relative difference of two varying pressures. In all cases, the region across which the field is applied must not be penetrated by any fluid with a different dielectric constant from that of the fluid originally between the two plates. U.S. Pat. No. 5,458,000 teaches the use of a second resonant sensor that is not affected by changes in pressure but responds to changes in temperature for use in temperature error compensation. This sensor will not operate in a viscous fluid. U.S. Pat. Nos. 5,528,939 and 5,939,635 teach the measurement of energy needed to move a resonant member near a stationary surface, where the damping losses are highly pressure dependent. This can only be used to measure a compressible working fluid, however, or the variation in damping force will not be evident. U.S. Pat. No. 5,808,210 teaches measurement of absolute pressure by measuring deformation either electrically or optically. While this sensor can be used for optical measurement of the membrane deflection, it cannot be used with an opaque working fluid.
It is an object of the present invention to provide an improved way to measure the absolute static pressure in a microfluidic device transporting a working fluid.
This object is achieved in a method of measuring absolute static pressure at one or more positions along the wall of a microfluidic device transporting a working fluid that is immiscible in a first selected gas environment, comprising the steps of:
(a) providing a first fluid conducting channel having an atmosphere provided by the first selected gas environment under a sealed environment and in communication with the microfluidic device at a first point of communication;
(b) providing a first visual scale adjacent to the first fluid conducting channel; and
(c) transporting the working fluid under pressure conducted by the microfluidic device into the first fluid conducting channel such that the volume transported into such first fluid conducting channel varies depending upon the absolute static pressure of the working fluid at the first point of communication, whereby the absolute static pressure at the first point of communication is visually determined depending on the position of the interface of the working fluid and the first selected gas environment in the first fluid conducting channel when compared with the first visual scale.
It is an advantage of the present invention to visually inspect the volume in a first fluid conducting channel in communication with the microfluidic device to readily determine the absolute static pressure in the first fluid conducting channel without the need for electrical elements or moving parts.
It is an advantage of the present invention to integrate the absolute static pressure sensor with nearly any existing microfluidic device without modification of the fabrication process of the microfluidic device.
It is an advantage of the present invention that the integration referred to above will not increase the fabrication cost of the pre-existing microfluidic device.
It is a further advantage of the present invention to measure absolute static pressure over a variety of pressure ranges at the same point of communication, thereby increasing the effective absolute static pressure range of the sensor.
It is a further advantage of the present invention to measure absolute static pressure at a precise location within a microfluidic device.